I brewed a partial mash imperial stout on Sunday. My target OG was 1.090 but because I tried to use too much grain compared to the water capacity of my brew kettle (BIAB method), I ended up only hitting 1.070 (had to add water back into the boil in order to hit post boil 2.5 gallons). Lesson learned for future brews about water to grain ratio and it's impact upon efficiency.

The problem is that before I took my OG reading, I had already rehydrated my 1.5 packs of S-05 yeast, per MrMalty's pitching recommendations on five gallons of 1.090 OG beer. So I ended up pitching 1.5 packs of yeast. But based on the 1.070 actual OG, I should have only pitched 1.2 packs.

My question then is whether I overpitched so much that I can expect strong off flavors to result (fortunately, a black as night stout should in theory mitigate some of that) or if my rate of overpitching was light enough where the beer won't end up tasting like a mess.

You have to significantly over pitch (on the order of 4-6x recommended) to have a large enough effect to taste on a homebrew scale. Greatest effect will be a smaller growth phase which might decrease growth related flavors, but with US-05 you aren't looking for those anyway. Belgians and Hefeweizens are more likely to suffer from overpitching decreased flavors than a stout.
You might consider sparging your grain bag in another pot to extract more sugars next time, there were probably a ton of gravity points left in the grain bag after a low water to grist ratio mash like that. You can always boil off excess volume and concentrate the wort if you end up with a lower than desired gravity wort that is a larger volume than needed.